1. Field of the Disclosure
A method used in a wireless communication system and related communication device are provided, and more particularly to, a method of handling call origination in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
For making a mobile origination call, the UE may determine the call type (e.g. circuited switch (CS) voice call or IP multimedia subsystem (IMS) voice call). Namely, the UE decides which call type has to be used before sending a request (e.g. RRC connection request) to the network. If the network supports the determined call type and accepts the request, the network may send an RRC Connection Setup message to the UE. The UE receives the RRC Connection Setup message and sends an RRC Setup complete message in response to the RRC Connection Setup message. If the network does not support the determined call type, the network may send a reject message (e.g. Service Reject message) to the UE. The UE may find another cell which supports the determined call type. If such cell is not found, the UE may indicate a failure to a user.
In the LTE system, levels of services which are provided by the network to the UE are defined as follows: (1) Normal service; (2) Limited service; (3) Operator service. When a UE is switched on, the UE selects a public land mobile network (PLMN) and camp on a “suitable cell” of the PLMN for the normal service. The suitable cell shall fulfill some requirements (basically detailed in 3GPP specification (3GPP TS 23.167), and thus omitted herein). The UE using the normal service may receive system information or registration area information from a public land mobile network (PLMN) as well as receive access stratum (AS) and non-access stratum (NAS) information. If the UE is registered to the network, the UE may receive paging and notification messages from the PLMN and initiate call setup for outgoing calls or the other actions. If the UE is unable to find any suitable cell to camp on, or a universal subscriber identity module (USIM) is not inserted into the UE, or if the location registration is failed, the UE attempts to camp on a cell irrespective of the PLMN, and enters the “limited service” state. In the limited service state, the UE can only make emergency calls.
IP multimedia subsystem (IMS) emergency calls are supported in the specification and the UE may initiate an IMS emergency call on the packet switch (PS) domain if the network supports the IMS emergency call service. An indication is provided to inform the UE that IMS emergency call services are supported. The indication is sent via NAS messaging for the UE using the normal service and/or via a broadcast control channel (BCCH) indicator for the UE using the limited service. The BCCH indicator is set to ‘support’ if any of the mobility management entities (MMES) in a non-shared environment or one of PLMNs in a shared network environment supports IMS emergency call services. Then, the eNB performs MME selection according to the PLMN selected by the UE. If at the time of the IMS emergency call origination, the UE has established a RRC (Radio Resource Control) connection with a core network that does not support IMS emergency calls, the UE should autonomously release the RRC connection and find a new cell that is capable of handling IMS emergency calls. However, the UE may not find any cell supporting IMS emergency call. In this situation, the IMS emergency call could be delayed. In addition, if the UE is operated in the normal service state, the UE performs a “Tracking Area Update” procedure to notify the core network of a geographical location change when selecting the new cell capable of handling the IMS emergency call in another PLMS. This may delay the IMS emergency call origination as well.
On the other hand, the UE may fail in originating the circuit switch (CS) emergency call in LTE system. In this situation, the UE can only camp on radio technologies (RATs) that provide access to the CS domain (e.g. GERAN or UTRAN) and disable capability of the serving RAT and indicates a failure to the user. Unfortunately, this limits the UE to the exclusive way for originating emergency call and turns down all possibilities of emergency call originations. And, the user is not willing to see the failure.
Besides, when a disaster (e.g. earthquake) occurs, there may be bunch of UEs making calls in the disaster area. The network cannot handle all calls simultaneously caused by the capacity limitation because the network is unable to perform loading balancing. When the UE makes a mobile origination call, the UE determines a call type (e.g. CS voice call or IMS voice call) before sending a request message to the network. When a bunch of UEs originate calls by the same call type, this may cause heavy traffic.